1. Field of the Invention
The present invention relates to a fixing device mounted on image forming apparatuses such as a copying machine, a printer, and a facsimile, and, more particularly to a fixing device for an image forming apparatus that quickly and highly accurately performs temperature control.
2. Description of the Background
In a fixing device of an induction heating system used in image forming apparatuses of an electrophotographic system such as a copying machine and a printer, in general, the surface temperature of a heat roller is detected and a result of the detection is fed back to an induction heating coil to perform temperature control for the fixing device. In the past, such temperature control for the fixing device is performed using, for example, a CPU that controls operations of a printer. On the other hand, when a heat capacity of the fixing device is small, the temperature of the fixing device instantaneously and widely fluctuates depending on a fixing condition and the like. Thus, when the temperature control for the fixing device is delayed, it is likely that fixing performance is adversely affected by the delay. Therefore, it is necessary to quickly perform feedback control for the fixing device.
However, when electric power supplied to the induction heating coil is feedback-controlled using the CPU to subject the fixing device to temperature control as in the past, depending on processing speed of the CPU, it is likely that the supplied electric power cannot be instantaneously controlled. Since the control is delayed, it is likely that a temperature ripple of the fixing device increases, resulting in overshoot of the fixing device. Further, it is likely that an optimum fixing temperature corresponding to an operation mode is not obtained and, in particular, temperature control in a high-speed image forming apparatus is difficult.
Therefore, as the fixing device of the induction heating system, there is a demand for development of a fixing device for an image forming apparatus that instantaneously feedback-controls the supply of electric power to an induction heating coil, maintains a stable fixing temperature even if a heat capacity of the fixing device is small, and obtains a high-quality fixing image.